The present invention relates to a zoom lens suitable for a compact camera which is subject to a small backfocus, particularly relates to a zoom lens for a compact camera with half an angle of view of approximately 37.degree. and with a high zoom ratio of 2.5 or more.
A variety of zoom lenses for compact cameras have been known heretofore. The zoom lenses consisting, of three groups or more with a zoom-ratio exceeding two times are categorized as follows:
(i) Four-lens group zoom lens system comprising, in order from the object side a positive lens group I, a negative lens group II, a stop diaphragm, a positive lens group III, and a negative lens group IV, all the lens groups being moved independent of each other toward the object- (this type includes a zoom lens where some of the four lens groups are moved in unison with each other). For example, such a zoom lens system is disclosed in Japanese patent laid-open publications No. SHO 63-43115, NO. SHO 63-159818, and No. SHO 63-157120.
(ii) Three-lens group zoom lens system comprising a positive lens group I, a negative lens group II (which has a negative lens group IIa disposed toward the object, a positive lens group IIb disposed toward the image, and a stop diaphragm therebetween and which may be equivalent to the lens group II and the lens group III of the four-lens group zoom system) and a negative lens group II (which s equivalent to the lens group IV of the above-described four-lens group zoom system), all the lens groups being moved independently of each other toward the object. For example, the zoom lens system are disclosed in Japanese patent laid-open publications No. SHO 63-15313 and NO. SHO 63-161423.
A four-lens group zoom system and a three-lens group zoom system whose zoom ratio is less than 2 (approximately, 1.5 to 1.6) are disclosed in Japanese patent laid-open publications Nos. SHO 60-57814 and SHO 62-78522, respectively. The arrangement of lens groups and a stop diaphragm of these two systems are the same as those in the system (i) and (ii).
(iii) Three-lens group zoom type lens comprising, in order from the object, a positive lens group I, a positive lens group II (which is the same as the system (ii) except that no stop diaphragm is used therein), a stop diaphragm after the group II negative lens group III disposed, all the lens groups being moved toward the object. For example, the zoom lens system was invented by the present inventor and (U.S. patent application Ser. No. 404,399 filed on Sept. 7, 1989, now U.S. Pat. No. 4,978,204).
(iv) Substantially four-lens group zoom system wherein the lens groups IIa and IIb are moved independently of each other in the system (iii). For example, the zoom lens is disclosed also in the foregoing U.S. affliction.
However, in the system (i), all the four lens groups should be moved independently of each other. Thus, a member of cams should be used in a restricted small space, resulting in complication in mechanism.
In the systems (i) and (ii), although the stop diaphragm is disposed between the lens group IIa and the lens group IIb (in the case of the four-lens group system, the stop diaphragm is disposed between the lens group II and the lens group III), since a shutter mechanism should be disposed between the two lens groups, which is likely to lead to production errors, the arrangement would become complicated so that it is difficult to ensure a high mechanical accuracy with case. In addition, since the optical sensitivity is high, it would be difficult to stabilize the optical performance. Moreover, this system could not make the camera compact.
To solve the above noted problems, there is provided a system as proposed in the foregoing U.S. application.
In the systems (i), (ii), (iii), and (iv), half the angle view on the short focal length side is approximately 30.degree.. Thus, the focal length on the telephoto side is mainly improved to ensure a desired telephoto effect. On the other hand, it is also necessary to improve the performance on the wide-angle side or talking scenic pictures. The users require wide angle, high zoom-ratio compact cameras rather than those whose half angle of view on the short focal length side is approximately 30.degree.. However, the cameras which satisfy these demands have not been provide heretofore.
In other words, the foregoing systems relate to telephoto type zoom lenses due to the short backfocus over the entire zoom range. In addition, since a wide angle, high zoom ratio zoom lens covering half the angle of view of approximately 37.degree. was not known. Thus, it is difficult to accomplish a wide angle ability and a high zoom ratio at the same time for telephoto type compact system.